Top couplings: ILC-B physics interplay Top couplings: ILC-B physics interplay TYL-FJPPL B physics TYL-FJPPL B physics February 20, 2015 LAL February 20, 2015 LAL F. Richard February François Richard LAL/Orsay
Introduction Top EW couplings direct measurements can only be carried at HE colliders like ILC and LHC This talk discusses indirect measurements at B factories and at LEP1/SLC (T observable) I will compare the accuracies which are reachable with these various set ups F. Richard February 20152
LHC ATLAS and CMS are reaching an accuracy of a few % on Vtb e.g. CMS gives |V tb |=0.998±0.0038(exp.)±0.016(theo.) If one assumes a SM CKM with V tb ~1 this is translated into a constraint on the WbLtL coupling which, under mild assumptions, can be related to ZtLtL and ZbLbL (measured precisely at LEP1) Knowing ZtLtL one can use the ttZ cross section to extract ZtRtR Unfortunately this cross section is known to ±35% and since ZtRtR²/ZtLtL²~0.2, this measurement is insensitive to ZtRtR and this will remain true for long since this measurement is systematics limited to ~5% which corresponds to an error of ~15% on ZtRtR F. Richard February 20153
ILC ILC produces top quarks through and Z exchange Using polarized beams one can separately extract Z axial and vector couplings and the photon coupling (neglecting tensorial terms) 4 observables are available : tt and AFBt for the two polarisations ILC operates at 500 GeV centre of mass well above tt threshold to avoid large coulombic QCD corrections and to keep good sensitivity to the axial term Statistical accuracies are overwhelmingly good, beyond theoretical present uncertainties, specially EW corrections F. Richard February 20154
Comparisons LHC ILC The very large errors at LHC are due to the sign uncertainty which is absent at ILC due to Z- interference The tensorial terms F2V is obtained assuming SM vectorial terms F2A can be independently measured by measuring CPV distributions I will concentrate on Z couplings F. Richard February 20155
Top coupling through loops The B factory cleanest mode seems to be: Depends on WbLtL and ZtLtL The T parameter also depends on top loops In both cases beware that additional BSM loops (heavy quarks, Z’) can also contribute F. Richard February 20156
EFT From one writes T= SU(2)L gives There are only 2 EFT parameters simply related to gL and gR Truly most general ? Looking at concrete BSM models, I believe that this not the case F. Richard February 20157
Results As expected B physics red band is almost horizontal The black ellipse is due to the very tight T constraint ILC errors which correspond to dgL/gL=1% and dgR/gR=2% are correlated (-0.8) Systematics are ignored so far LHC and B physics are inaccurate for gR (ignoring T) F. Richard February
Comments It can be shown that the SMALL residual sensitivity of B physics to ZtRtR is due to the large top mass These comparisons are useful to evaluate the sensitivity of the various set ups but do not give the whole picture New physics can provide additional diagrams which can influence differently the direct and indirect measurements As an example one can have a Z’ which is not flavor diagonal and contributes to B->µµ but not to ILC couplings At ILC a Z’ can be observed through its propagator with an effect going like ~s/M²z’ (energy dependent) F. Richard February 20159
Conclusions A precise and model independent determination of top EW couplings requires ILC with polarised beams The high accuracy achievable allows to reach very large BSM scales, beyond LHC (e.g. KK exchange in RS) but beware the theory errors The T parameter seems to play a crucial part but this constraint has to be taken with a grain of salt given that it it is far from clear that the EFT formalism covers all BSM scenarios (counterexample in Little Higgs model) B factories can add a crucial piece of information since these results also depend on loops which can be affected differently by BSM exchanges (e.g. flavour non diagonal Z’ in Bs->µµ) F. Richard February
F. Richard February BACK UP SLIDES
Expected deviations F. Richard February
Little Higgs model F. Richard February
Future F. Richard February
Ref F. Richard February
T/ 1 b F. Richard February
EFT F. Richard February